Chapter Questions
Refer to Example 3.2. From the histogram for investment $\mathrm{A}$, estimate the following probabilities.a. $P(X>45)$b. $P(10<X<40)$c. $P(X<25)$d. $P(35<X<65)$
Refer to Example 3.2. Estimate the following from the histogram of the returns on investment $B$.a. $P(X>45)$b. $P(10<X<40)$c. $P(X<25)$d. $P(35<X<65)$
Refer to Example 3.3. From the histogram of the marks, estimate the following probabilities.a. $P(55<X<80)$b. $P(X>65)$c. $P(X<85)$d. $P(75<X<85)$
A random variable is uniformly distributed between 5 and 25.a. Draw the density function.b. Find $P(X>25)$.c. Find $P(10<X<15)$.d. Find $P(5.0<X<5.1)$.
A uniformly distributed random variable has minimum and maximum values of 20 and 60 , respectively.a. Draw the density function.b. Determine $P(35<X<45)$.c. Draw the density function including the calculation of the probability in part (b).
The amount of time it takes for a student to complete a statistics quiz is uniformly distributed between 30 and 60 minutes. One student is selected at random. Find the probability of the following events.a. The student requires more than 55 minutes to complete the quiz.b. The student completes the quiz in a time between 30 and 40 minutes.c. The student completes the quiz in exactly 37.23 minutes.
Refer to Exercise 8.6. The professor wants to reward (with bonus marks) students who are in the lowest quarter of completion times. What completion time should he use for the cutoff for awarding bonus marks?
Refer to Exercise 8.6. The professor would like to track (and possibly help) students who are in the top $10 \%$ of completion times. What completion time should he use?
The weekly output of a steel mill is a uniformly distributed random variable that lies between 110 and 175 metric tons.a. Compute the probability that the steel mill will produce more than 150 metric tons next week.b. Determine the probability that the steel mill will produce between 120 and 160 metric tons next week.
Refer to Exercise 8.9. The operations manager labels any week that is in the bottom $20 \%$ of production a "bad week." How many metric tons should be used to define a bad week?
A random variable has the following density function.$$f(x)=1-.5 x \quad 0<x<2$$a. Graph the density function.b. Verify that $f(x)$ is a density function.c. Find $P(X>1)$.d. Find $P(X<.5)$.e. Find $P(X=1.5)$.
The following function is the density function for the random variable $X$ :$$f(x)=\frac{x-1}{8} \quad 1<x<5$$a. Graph the density function.b. Find the probability that $X$ lies between 2 and 4 .c. What is the probability that $X$ is less than 3?
The following density function describes the random variable $X$.$$f(x)=\left\{\begin{array}{cc}\frac{x}{25} & 0<x<5 \\\frac{10-x}{25} & 5<x<10\end{array}\right.$$
The following is a graph of a density function.a. Determine the density function.b. Find the probability that $X$ is greater than 10 .c. Find the probability that $X$ lies between 6 and 12 .
P(Z<1.50)$
P(Z<1.51)$
P(Z<1.55)$
P(Z<-1.59)$
P(Z<-1.60)$
$P(Z<-2.30)$
P(-1.40<Z<.60)$
P(Z>-1.44)$
P(Z<2.03)$
$P(Z>1.67)$
$P(Z<2.84)$
P(1.14<Z<2.43)$
$P(-0.91<Z<-0.33)$
$P(Z>3.09)$
$P(Z>0)$
$P(Z>4.0)$
Find $z_{.02}$.
Find $z_{.045}$.
Find $z_{.20}$.x
$X$ is normally distributed with mean 100 and standard deviation 20. What is the probability that $X$ is greater than 145 ?
$X$ is normally distributed with mean 250 and standard deviation 40 . What value of $X$ does only the top $15 \%$ exceed?
$X$ is normally distributed with mean 1,000 and standard deviation 250 . What is the probability that $X$ lies between 800 and 1,100 ?
$X$ is normally distributed with mean 50 and standard deviation 8 . What value of $X$ is such that only $8 \%$ of values are below it?
The long-distance calls made by the employees of a company are normally distributed with a mean of 6.3 minutes and a standard deviation of 2.2 minutes. Find the probability that a calla. lasts between 5 and 10 minutes.b. lasts more than 7 minutes.c. lasts less than 4 minutes.
Refer to Exercise 8.38. How long do the longest $10 \%$ of calls last?
The lifetimes of lightbulbs that are advertised to last for 5,000 hours are normally distributed with a mean of 5,100 hours and a standard deviation of 200 hours. What is the probability that a bulb lasts longer than the advertised figure?
Refer to Exercise 8.40. If we wanted to be sure that $98 \%$ of all bulbs last longer than the advertised figure, what figure should be advertised?
Travelbyus is an Internet-based travel agency wherein customers can see videos of the cities they plan to visit. The number of hits daily is a normally distributed random variable with a mean of 10,000 and a standard deviation of 2,400 .a. What is the probability of getting more than 12,000 hits?b. What is the probability of getting fewer than 9,000 hits?
Refer to Exercise 8.42. Some Internet sites have bandwidths that are not sufficient to handle all their traffic, often causing their systems to crash. Bandwidth can be measured by the number of hits a system can handle. How large a bandwidth should Travelbyus have in order to handle $99.9 \%$ of daily traffic?
A new gas-electric hybrid car has recently hit the market. The distance traveled on 1 gallon of fuel is normally distributed with a mean of 65 miles and a standard deviation of 4 miles. Find the probability of the following events.a. The car travels more than 70 miles per gallon.b. The car travels less than 60 miles per gallon.c. The car travels between 55 and 70 miles per gallon.
The top-selling Red and Voss tire is rated 70,000 miles, which means nothing. In fact, the distance the tires can run until they wear out is a normally distributed random variable with a mean of 82,000 miles and a standard deviation of 6,400 miles.a. What is the probability that a tire wears out before 70,000 miles?b. What is the probability that a tire lasts more than 100,000 miles?
The heights of 2-year-old children are normally distributed with a mean of 32 inches and a standard deviation of 1.5 inches. Pediatricians regularly measure the heights of toddlers to determine whether there is a problem. There may be a problem when a child is in the top or bottom $5 \%$ of heights. Determine the heights of 2-year-old children that could be a problem.
Refer to Exercise 8.46 . Find the probability of these events.a. A 2-year-old child is taller than 36 inches.b. A 2-year-old child is shorter than 34 inches.c. A 2-year-old child is between 30 and 33 inches tall.
University and college students average 7.2 hours of sleep per night, with a standard deviation of $40 \mathrm{~min}-$ utes. If the amount of sleep is normally distributed, what proportion of university and college students sleep for more than 8 hours?
Refer to Exercise 8.48. Find the amount of sleep that is exceeded by only $25 \%$ of students.
The amount of time devoted to studying statistics each week by students who achieve a grade of $\mathrm{A}$ in the course is a normally distributed random variable with a mean of 7.5 hours and a standard deviation of 2.1 hours.a. What proportion of $\mathrm{A}$ students study for more than 10 hours per week?b. Find the probability that an A student spends between 7 and 9 hours studying.c. What proportion of A students spend fewer than 3 hours studying?d. What is the amount of time below which only $5 \%$ of all A students spend studying?
The number of pages printed before replacing the cartridge in a laser printer is normally distributed with a mean of 11,500 pages and a standard deviation of 800 pages. A new cartridge has just been installed.a. What is the probability that the printer produces more than 12,000 pages before this cartridge must be replaced?b. What is the probability that the printer produces fewer than 10,000 pages?
Refer to Exercise 8.51. The manufacturer wants to provide guidelines to potential customers advising them of the minimum number of pages they can expect from each cartridge. How many pages should it advertise if the company wants to be correct $99 \%$ of the time?
Battery manufacturers compete on the basis of the amount of time their products last in cameras and toys. A manufacturer of alkaline batteries has observed that its batteries last for an average of 26 hours when used in a toy racing car. The amount of time is normally distributed with a standard deviation of 2.5 hours.
Because of the relatively high interest rates, most consumers attempt to pay off their credit card bills promptly. However, this is not always possible. An analysis of the amount of interest paid monthly by a bank's Visa cardholders reveals that the amount is normally distributed with a mean of $$\$ 27$$ and a standard deviation of $$\$ 7$$.a. What proportion of the bank's Visa cardholders pay more than $$\$ 30$$ in interest?b. What proportion of the bank's Visa cardholders pay more than $$\$ 40$$ in interest?c. What proportion of the bank's Visa cardholders pay less than $$\$ 15$$ in interest?d. What interest payment is exceeded by only $20 \%$ of the bank's Visa cardholders?
It is said that sufferers of a cold virus experience symptoms for 7 days. However, the amount of time is actually a normally distributed random variable whose mean is 7.5 days and whose standard deviation is 1.2 days.a. What proportion of cold sufferers experience fewer than 4 days of symptoms?b. What proportion of cold sufferers experience symptoms for between 7 and 10 days?
How much money does a typical family of four spend at a McDonald's restaurant per visit? The amount is a normally distributed random variable with a mean of $$\$ 16.40$$ and a standard deviation of $$\$ 2.75$$.a. Find the probability that a family of four spends less than $$\$ 10$$.b. What is the amount below which only $10 \%$ of families of four spend at McDonald's?
The final marks in a statistics course are normally distributed with a mean of 70 and a standard deviation of 10 . The professor must convert all marks to letter grades. She decides that she wants $10 \% \mathrm{~A}$ ', $30 \%$ B's, $40 \% \mathrm{C}$ 's, $15 \% \mathrm{D}$ 's, and $5 \%$ F's. Determine the cutoffs for each letter grade.
Mensa is an organization whose members possess IQs that are in the top $2 \%$ of the population. It is known that IQs are normally distributed with a mean of 100 and a standard deviation of 16 . Find the minimum IQ needed to be a Mensa member.
According to the 2001 Canadian census, university-educated Canadians earned a mean income of $$\$ 61,823$$. The standard deviation is $$\$ 17,301$$. If
The census referred to in the previous exercise also reported that college-educated Canadians earn on average $$\$ 41,825$$. Suppose that incomes are normally distributed with a standard deviation of $$\$ 13,444$$. Find the probability that a randomly selected college-educated Canadian earns less than $$\$ 45,000$$.
The lifetimes of televisions produced by the Hishobi Company are normally distributed with a mean of 75 months and a standard deviation of 8 months. If the manufacturer wants to have to replace only $1 \%$ of its televisions, what should its warranty be?
According to the Statistical Abstnuct of the United States, 2012 (Table 721), the mean family net worth of families whose head is between 35 and 44 years old is approximately $$\$ 325,600$$. If family net worth is normally distributed with a standard deviation of $$\$ 100,000$$, find the probability that a randomly selected family whose head is between 35 and 44 years old has a net worth greater than $$\$ 500,000$$.
A retailer of computing products sells a variety of computer-related products. One of his most popular products is an HP laser printer. The average weekly demand is 200 . Lead time for a new order from the manufacturer to arrive is 1 week. If the demand for printers were constant, the retailer would reorder when there were exactly 200 printers in inventory. However, the demand is a random variable. An analysis of previous weeks reveals that the weekly demand standard deviation is 30 . The retailer knows that if a customer wants to buy an HP laser printer but he has none available, he will lose that sale plus possibly additional sales. He wants the probability of running short in any week to be no more than $6 \%$. How many HP laser printers should he have in stock when he reorders from the manufacturer?
The demand for a daily newspaper at a newsstand at a busy intersection is known to be normally distributed with a mean of 150 and a standard deviation of 25. How many newspapers should the newsstand operator order to ensure that he runs short on no more than $20 \%$ of days?
Every day a bakery prepares its famous marble rye. A statistically savvy customer determined that daily demand is normally distributed with a mean of 850 and a standard deviation of 90 . How many loaves should the bakery make if it wants the probability of running short on any day to be no more than $30 \%$ ?
Refer to Exercise 8.65. Any marble ryes that are unsold at the end of the day are marked down and sold for half-price. How many loaves should the bakery prepare so that the proportion of days that result in unsold loaves is no more than $60 \%$ ?
Refer to Exercise 7.57. Find the probability that the project will take more than 60 days to complete.
The mean and variance of the time to complete the project in Exercise 7.58 was 145 minutes and 31 minutes ${ }^2$. What is the probability that it will take less than 2.5 hours to overhaul the machine?
The annual rate of return on a mutual fund is normally distributed with a mean of $14 \%$ and a standard deviation of $18 \%$.a. What is the probability that the fund returns more than $25 \%$ next year?b. What is the probability that the fund loses money next year?
In Exercise 7.64, we discovered that the expected return is .1060 and the standard deviation is .1456. Working with the assumption that returns are normally distributed, determine the probability of the following events.a. The portfolio loses money.b. The return on the portfolio is greater than $20 \%$.
The random variable $X$ is exponentially distributed with $\lambda=3$. Sketch the graph of the distribution of $X$ by plotting and connecting the points representing $f(x)$ for $x=0, .5,1,1.5$, and 2 .
$X$ is an exponential random variable with $\lambda=.25$. Sketch the graph of the distribution of $X$ by plotting and connecting the points representing $f(x)$ for $x=0,2,4,6,8,10,15,20$.X
Let $X$ be an exponential random variable with $\lambda=.5$. Find the following probabilities.a. $P(X>1)$b. $P(X>.4)$c. $P(X<.5)$d. $P(X<2)$
$X$ is an exponential random variable with $\lambda=.3$. Find the following probabilities.a. $P(X>2)$b. $P(X<4)$c. $P(1<X<2)$d. $P(X=3)$
The production of a complex chemical needed for anticancer drugs is exponentially distributed with $\lambda=6$ kilograms per hour. What is the probability that the production process requires more than 15 minutes to produce the next kilogram of drugs?
The time between breakdowns of aging machines is known to be exponentially distributed with a mean of 25 hours. The machine has just been repaired. Determine the probability that the next breakdown occurs more than 50 hours from now.
When trucks arrive at the Ambassador Bridge, each truck must be checked by customs agents. The times are exponentially distributed with a service rate of 10 per hour. What is the probability that a truck requires more than 15 minutes to be checked?
A bank wishing to increase its customer base advertises that it has the fastest service and that virtually all of its customers are served in less than 10 minutes. A management scientist has studied the service times and concluded that service times are exponentially distributed with a mean of 5 minutes. Determine what the bank means when it claims "virtually all" its customers are served in less than 10 minutes.
Toll booths on the New York State Thruway are often congested because of the large number of cars waiting to pay. A consultant working for the state concluded that if service times are measured from the time a car stops in line until it leaves, service times are exponentially distributed with a mean of 2.7 minutes. What proportion of cars can get through the toll booth in less than 3 minutes?
The manager of a gas station has observed that the times required by drivers to fill their car's tank and pay are quite variable. In fact, the times are exponentially distributed with a mean of 7.5 minutes. What is the probability that a car can complete the transaction in less than 5 minutes?
Because automatic banking machine (ABM) customers can perform a number of transactions, the times to complete them can be quite variable. A banking consultant has noted that the times areexponentially distributed with a mean of $125 \mathrm{sec}-$ onds. What proportion of the ABM customers take more than 3 minutes to do their banking?
The manager of a supermarket tracked the amount of time needed for customers to be served by the cashier. After checking with his statistics professor, he concluded that the checkout times are exponentially distributed with a mean of 6 minutes. What proportion of customers require more than $10 \mathrm{~min}$ utes to check out?
Use the $t$ table (Table 4 ) to find the following values of $t$.a. $t_{.10,15}$b. $t_{.10,23}$c. $t_{.025,83}$d. $t_{.05,195}$
Use the $t$ table (Table 4) to find the following values of $t$.a. $t_{.005,33}$b. $t_{.10,600}$c. $t_{, 05,4}$d. $t_{.01,20}$
Use a computer to find the following values of $t$.a. $t_{.10,15}$b. $t_{\cdot 10,23}$c. $t_{.025,83}$d. $t_{.05,195}$
Use a computer to find the following values of $t$.a. $t_{.05,143}$b. $t_{.01,12}$c. $t_{025, \infty}$d. $t_{.05,100}$
Use a computer to find the following probabilities.a. $P\left(t_{64}>2.12\right)$c. $P\left(t_{159}>1.33\right)$b. $P\left(t_{27}>1.90\right)$d. $P\left(t_{550}>1.85\right)$
Use a computer to find the following probabilities.a. $P\left(t_{141}>.94\right)$c. $P\left(t_{1000}>1.96\right)$b. $P\left(t_{421}>2.00\right)$d. $P\left(t_{\mathrm{B} 2}>1.96\right)$
Use the $\chi^2$ table (Table 5) to find the following values of $x^2$.a. $x_{10,5}^2$b. $\chi_{.01,100}^2$c. $x_2^2 .95,18$d. $x_{.99,60}^2$
Use the $\chi^2$ table (Table 5 ) to find the following values of $x^2$.a. $x_1^2 \cdot 90,26$b. $\chi_{.01,30}^2$c. $x_{.10,1}^2$d. $\chi_{999,80}^2$
Use a computer to find the following values of $\chi^2$.a. $x_{.25,66}^2$b. $\chi_{.40,100}^2$c. $x_{50,17}^2$d. $x_{.10,17}^2$
Use a computer to find the following values of $x^2$.a. $\chi^2 .99,55$b. $\chi_{.05,800}^2$c. $x_{.99,43}^2$d. $x_{.10,233}^2$
Use a computer to find the following probabilities.a. $P\left(x_{73}^2>80\right)$b. $P\left(x_{200}^2>125\right)$c. $P\left(x_{88}^2>60\right)$d. $P\left(\chi_{1000}^2>450\right)$
Use a computer to find the following probabilities.a. $P\left(x_{250}^2>250\right)$c. $P\left(x_{600}^2>500\right)$b. $P\left(x_{36}^2>25\right)$d. $P\left(x_{120}^2>100\right)$
Use the $F$ table (Table 6 ) to find the following values of $F$.a. $F_{.05,3,7}$b. $F_{.05,7,3}$c. $F_{.025,5,20}$ d. $F_{.01,12,60}$
Use the $F$ table (Table 6 ) to find the following values of $F$.a. $F_{.025,8,22}$c. $F_{.01,9,18}$b. $F_{.05,20,30}^{025,8,22}$d. $F_{.025,24,10}^{01,9,18}$
Use a computer to find the following values of $F$.a. $F_{05,70,70}$c. $F_{025,36,50}$b. $F_{.01,45,100}^{05,70,70}$d. $F_{.05,500,500}^{025,36,50}$
Use a computer to find the following values of $F$.a. $F_{.01,100,150}$c. $F_{.01,11,33}$b. $F_{.05,25,125}$d. $F_{.05,300,800}^{01,1,33}$
Use a computer to find the following probabilities.a. $P\left(F_{7,20}>2.5\right)$c. $P\left(F_{34,62}>1.8\right)$b. $P\left(F_{18,63}>1.4\right)$d. $P\left(F_{200,400}>1.1\right)$
Use a computer to find the following probabilities.a. $P\left(F_{600,800}>1.1\right)$b. $P\left(F_{35,100}>1.3\right)$c. $P\left(F_{66,148}>2.1\right)$d. $P\left(F_{17,37}>2.8\right)$